1. Field of the Invention
The present invention relates to laser scanning optical systems, and more particularly to laser scanning optical systems having an SOS sensor for providing a synchronizing signal.
2. Description of the Related Art
Conventional laser scanning optical systems include an SOS sensor for providing a synchronizing signal commanding the start of modulation of the laser beam at a specified position without any alteration with respect to the subscanning direction. To assure the SOS sensor of good detecting performance, it is desirable to dispose the sensor at a position where the laser beam is concentrated, so that the sensor is provided usually at a position equivalent to the image receiving surface on which the laser beam is to be concentrated. With laser printers, however, it is not always easy to position the SOS sensor in an equivalent with the surface of the photosensitive member serving as the image receiving surface owing to the interference with other image forming elements.
To overcome this problem, Japanese Laid-Open Paid Application No. 54-140548 discloses an optical disposed at a position beyond a position equivalent to the surface of a photosensitive member, and an optical system comprising a condenser lens for converging a laser beam on the SOS sensor. Nevertheless, the disclosed optical system is merely so adapted as to converge the laser beam with the condenser lens, so that the laser beam projected is likely to deflect from the SOS sensor owing to an inclination of the axis of the polygonal mirror or to errors involved in installing optical elements. FIG. 1 shows an arrangement in which a condenser lens L is interposed between an SOS sensor S and a position A which is equivalent to the photosensitive member and at which a laser beam is concentrated. If the laser beam deflects from the position A by an amount with respect to the subscanning direction, the amount of deflection at the position where the SOS sensor is disposed is expressed by the following equation. EQU .delta.'=b/a.multidot..delta.
Thus, the presence of the condenser lens multiplies the amount of deflection .delta. by the magnification (b/a). Accordingly, the disclosed technique has a problem in that there is an increased likelihood of the laser beam deflecting from the light-receiving surface of the SOS sensor and becoming undetectable.
Japanese Laid-Open Patent Application No. 54-140558 discloses an optical system wherein a laser beam is caused to impinge on a polygonal mirror more than once and to impinge on an SOS sensor at an increased scanning speed so as to diminish synchronization errors. However, the disclosed optical system has a complex construction, while the SOS sensor is equivalent to the photosensitive member in position.